Aurora Tornado Rated EF-1

When we were looking at the damage last night, this is exactly what we thought. An EF-1 tornado will have wind speeds from 86 to 110 miles per hour. Recent notable tornadoes in Colorado were Holly in March of 2007 and Windsor in May of 2008. Both of these tornadoes were classified as EF-3 with wind speeds of 136 to 165 mph. Below is some good information about the EF Scale, courtesy of the Storm Prediction Center. It talks about the transition from the Fujita to the Enhanced Fujita Scale and why the switch was made. We will have more information about the Denver twisters later this week.

Dr. T. Theodore Fujita first introduced The Fujita Scale in the SMRP Research Paper, Number 91, published in February 1971 and titled, "Proposed Characterization of Tornadoes and Hurricanes by Area and Intensity". Fujita revealed in the abstract his dreams and intentions of the F-Scale. He wanted something that categorized each tornado by intensity and area. The scale was divided into six categories:

F0 (Gale)

F1 (Weak)

F2 (Strong)

F3 (Severe)

F4 (Devastating)

F5 (Incredible)

Dr. Fujita's goals in his research in developing the F-Scale were

categorize each tornado by its intensity and its area

estimate a wind speed associated with the damage caused by the tornado

Dr. Fujita and his staff showed the value of the scale's application by surveying every tornado from the Super Outbreak of April 3-4, 1974. The F-Scale then became the mainstay to define every tornado that has occurred in the United States. The F-Scale also became the heart of the tornado database that contains a record of every tornado in the United States since 1950.

Figure 1: Number of tornadoes per year, 1950-2004 (click image to enlarge)

The United States today averages 1200 tornadoes a year. The number of tornadoes increased dramatically in the 1990s as the modernized National Weather Service installed the Doppler Radar network. The National Weather Service modernization also began the Warning Coordination Meteorologist program increasing partnerships with media and Emergency Management across the United States. This program also initiated the training of storm spotters across the County Warning Area of each Weather Forecast Office. With more people trained to relay information on storm activity to the Weather Forecast Office and improved communication and digital technology, more tornadoes could be reported.

While the Super Outbreak of tornadoes was the spring board for the F-Scale, it was the Jarrell, TX tornado of May 27, 1997 and the Oklahoma City/Moore, OK tornado of May 3, 1999 that brought to the forefront the problem that maybe the wind estimates were too high in the F-Scale. Meteorologists, Emergency Managers and Engineers convened on Moore, OK to study the weaknesses in the structures destroyed by the tornado of May 3, 1999. The findings can be found in the document FEMA 342, Building Assessment Report, Midwest Tornadoes of May 3, 1999, Observations, Recommendations and Technical Guidance. That document can be found here. Engineers claim that many homes are rated to withstand winds to 100 mph. Therefore, the question was raised that if a tornado has over 200 mph winds, how can the structure reveal this estimate when much of it is gone?

The Fujita Scale

The Fujita Scale is a well known scale that uses damage caused by a tornado and relates the damage to the fastest 1/4-mile wind at the height of a damaged structure. Fujita's scale was designed to connect smoothly the Beaufort Scale (B) with the speed of sound atmospheric scale, or Mach speed (M). Fujita explains explicitly that "F-scale winds are estimated from structural and/or tree damage, the estimated wind speed applies to the height of the apparent damage above the ground." Figure 1 shows graphically the relationship between the three scales.

Figure 2: Fujita's smoothly derived relationship of the F-Scale with the Beaufort Scale and the Mach Scale as explained to the right.
The Beaufort Scale is defined by the Glossary of of Meteorology (AMS) as a system of estimating and reporting wind speeds numerically from 0 (calm) to 12 (hurricane). The Mach scale is the speed of sound in the atmosphere. (click image to enlarge)

if the 3/4-mile wide tornado does not hit any structures, what F-scale should be assigned?

Subject to bias

Based on the worst damage (even if it is one building or house)

Overestimates wind speeds greater than F3

And the F-Scale has had its misuses over the years:

Too much reliance on the estimated wind speeds

Oversimplification of the damage description

Judge the F-scale by the appearance of the tornado cloud

Unrecognizing weak structures

mobile homes

modified homes

Fujita recognized that improvement was necessary. He published his memoirs called Mystery of Severe Storms in 1992 updating the Fujita Tornado Scale to include an estimate of f-scale damage then selecting the F-scale as a combination of f-scales and types of structural damage.

Figure 3: The "Modified" Fujita Scale. (click image to enlarge)

For example, if a tornado knocks down the walls of an area of homes. If it is determined that the walls collapsed, then the damage assigned is F3. If it is a brick home, then that lowers the damage to F2. Then, according to the table since it was a brick structure, then you +1 making the rating F3.

The Enhanced Fujita Scale

When the committee met to develop the Enhanced Fujita Scale (see original document) one point was made very clear: it must continue to support and maintain the original tornado database.; In other word, there must be some conformity to that of the F-Scale that is listed in the database. Other ideas were agreed to including:

Consistent Assessment of Damage

enhance description of damage with examples and photos

include not only structures, but also vegetation

base damage assignment on more than one structure, if available

develop a PC-based expert system

develop training materials

Data Collection

maintain current tornado database

surveys should include additional data

mean and maximum damage path width

basis for damage assignment

latitude/longitude of where the path began and ended

number of hours spent on the damage survey

names of survey team member(s)

When using the EF-Scale to determine the tornado's EF-rating, begin with the 28 Damage Indicators. Each one of these indicators have a description of the typical construction for that category of indicator. Then, the next step is to find the Degree of Damage (DOD). Each DOD in each category is given and expected estimate of wind speed, a lower bound of wind speed and an upper bound of wind speed.

Let's take the earlier example, a tornado moves through a neighborhood and walls are knocked down of an area of homes. Here the Damage indicator would be #2, One or Two Family Residences (FR12). The typical construction for this fits being a brick veneer siding home. The DOD would be a 9, most walls collapsed in bottom floor. Thus, the estimated winds would be 127 - 178 mph with the expected wind speed of 152 mph. Now, taking this number to the EF-Scale, the damage would be rated EF-3 with winds between 136 - 165 mph.

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